As resonators for use in high frequency bands and microwave bands, a quarter-wavelength or half-wavelength uniform transmission line resonator has been mainly used. In recent years, as indicated by Non-Patent Document 1, a stepped-impedance resonator (hereinafter, referred to also as “SIR”) configured by a plurality of transmission lines having different line impedances has been utilized for the purpose of realizing a small size, spurious suppression or various coupling systems.
The SIR includes various structures. As its typical example, a one-end-opened and the other-end-shorted SIR of a quarter-wavelength type and an open-ended SIR of a half-wavelength type are proposed. As indicated by Non-Patent Documents 2 and 3, since the quarter-wavelength SIR makes it possible to achieve the smallest size, it has been conventionally developed and put into practical use actively. Moreover, in recent years, because of the development of LTCC (Low Temperature Co-fired Ceramics) processing technology, many quarter-wavelength SIR's have been utilized as filters for use in microwave band radio systems as shown in Patent Literature 1.